distance prt

8/6/2019 Distance Prt

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EG/96/822

FACULTEIT DER ELEKTROTECHNIEK

Vakgroep Elektrische Energietechniek

Testing of distance protection systemby using Ol\1lCRON and EMTP-simulations.

E.f. WierengaEG /96/822.A

De Faculteit Elektrotechniek van deTechnische Universiteit Eindhoven aanvaardtgeen verantwoordelijkheid voor de inhoudvan stage- en afstudeerverslagen.

Afstudeerwerk verricht o.l.v.:Prof.dr.-ing. H. RijantoEindhoven, augustus 1996.

TECHNISCHE UNIVERSITEIT EINDHOVEN


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5ummaryTo develop a distance protection it is necessary to test its functionality in electrical systems.To test the hardware of the distance protection the static behaviour is examined by the testappliance MICRN. For testing the dynamic behaviour also the simulation programElectro Magnetic Transient Program (EMTP) is used. With this program transient voltagesand currents for different fault situations like the electrical arc can be calculated.For testing the static and the dynamic behaviour of the distance protection the respond aftera fault inception and determination of the fault location will be observed. The results are:Statie test In the starting unit there is equivocality in the setting voltage dependency. For several earth factors the time-impedance characteristic has a maximum deviationofSO %.

If the setting of the border in zone 1 is 0.1 n the maximum deviation of the border inthis zone is 2.8 n.

Dynamic test The DC-component has no influences on the behaviour of the distance protection During saturation of the current transformer or during an electrical arc at the faultlocation the distance protection locates the fault further on.With these results it can be concluded that for several earth factors and settings of the zonesthe deviation of determining the impedance does not comply with the requirements of 2%.Transients caused by saturation of the current transformer and the electrical arc tends todetermine the fault location further on.The development of the distance protection is still in process and therefore the next pointsare recommendable: Use equivalency in the setting of voltage dependency. Execute the entire test procedure for the latest hardware of the distance protection.

An important point of attention is comparing the deviations abovewith test resultsof the latest hardware. Extend the test by examining the auto reclosing of the distance protection.


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Testing of a distance protection system

Contents

2IntroductionPrinciple of distance protection2.1 Starting unit2.2 Measuring unit

4568

3 Performance of distance protection at special fault conditions 233.1 Current with OC-component 233.2 Saturation of current transformers 243.3 Electrical arc 294

5

67

Test of distance protection under steady state condition4.1 Setting regular test distance protection4.2 Test starting unit4.3 Test measuring unit4.4 Test results under steady state conditionTest of distance protection under transient condition5.1 Fault currentwith OC-component5.2 Saturation of current transformer5.3 Electrical arc at fault location5.4 Change of current direction5.5 Cross-country fault5.6 Test results under transient conditionConclusions and recommendationsReferences

323233414849525356586063

64

65Appendix A

Method of symmetrical componentsAppendix BTable phase selectionAppendixCTest equipment MICRN

66

68

70

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1 IntroductionThe working group transient analyse of the department EVT has drawn u p a specification todevelop a distance protection. The distance protection must be able to function i n m ed iu mvoltage systems with different neutral point earthing.To guarantee th e functionality of the distance protection the static an d dynamic behaviour ofthe distance protection s ho ul d b e tested for different system fault conditions.To test the hardware of the distance protection the static behaviour is examined by the testappliance MICRN. For testing the dynamic behaviour also the simulation programElectro Magnetic Transient Program (EMTP) is used. With this program transient voltagesan d currents for different fault situations like the electrical arc can be calculated.In chapter 2 the principle of the distance protection is set out. The accent is put on theprinciple of the starting unit for detecting the system fault and the measuring unit tocalculate the fault impedance an d also the direction.Chapter 3 examines three special fault situations in the system where the distance protectionmay not locate the fault location properly. The accent is put on the transients at the faultlocation, caused by these three fault situations.In the chapters 4 an d 5 the static an d the dynamic behaviour of the distance protection willbe tested. In both cases the determination of th e fault location and the programme ofisolating the faulted line will be observed.

Introduction -4 -


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unitII11II111111

_,


2 Principle of distance protectionTo perform the main and back-up protection without communication connections mostlythe distance protection is used. When a fault occurs on a transmission line, i t is necessary todetect the location of the fault in order to trip circuit-breakers at each end of the faulted linesection, and thus isolate that section from the power system. The fault location can bedetermined by measuring the impedance of the faulted conductors between the protectionlocation and the fault. This impedance is namely directly proportional to the corresponding'distance' from protection location to fault location. The region where the fault occurs in thesame line connection as the protection location is called the protection zone. The regionoutside the far-end of the line-connection, passing the substation, is then called the back-upzone. Figure 2.1 shows the principle operation of the distance protection.

u- - - - - - - - - - - - - - I

III

Starting unit IIIIIII Measuring

1------- - - - - - ~ - - - - - -11 Ph ase selectien unitII U IIIII

Timing unit I Impedance unit Directien unitIII------ ----------- - -

.I Tripping unit Ll I

Trip-cemmandFigure 2.1 Bloek-diagram of the principle operation of distance protection

As soon as a fault occurs, the starting unit has to respond and it will start the timing unit andthe phase-selection unit. The phase-selection unit provides the impedance unit theappropriate currents and voltages. Then the measurement quantities are used by theimpedance unit to determine the impedance between the protection location and the faultand by the direction unit to determine the direction of the energyflow.

-5 -Principle of distance protection


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Finally in the tripping unit the results of all units are combined and, depending on thedistance and the direction, it generates a trip comrnand to the circuit-breaker. The trippingdepends on the fault place. I f the fault is not far away, within the so-called protection zone,the Tripping time will be fast. Faults passing the protection zone will be covered by theback-up protection and the tripping will be delayed.In the following the starting and measuring unit will he explained in detail.

2.1 Storting unitThe starting uni t has to detect which phase(s) is (are) affected during a system fault. Forcases, where the fault current does not exceed the load current, for instance in case of backup protection the starting unit has also to genera te appropriate signais. In this case it isnecessary to have a voltage dependency. In figure 2.2 the so-called overcurrent startingfunctions with voltage dependency is illustrated.

UIU"t

IF> '> __ 111"Figure 2.2 Overcurrent with voltage dependency

If the current is between IF> and I> the starting function will only be activated, i f the voltageis below UF there will be no voltagedependency. The logical diagramwith voltage dependency is given in figure 2.3.



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Testing of a distance protection system-R (I-s (I-T (I-E (IE

Option LO Optien ISO l\ Optien IE>IF>

r - --''JT & START-RI-- + - I-- ~ I-- + & ,----- -- I--r---- & -'---- ' -UF< -.-----'JT r---- & START-SI--t--- r---- f-- ~ - & f--J f - - -f--t--- & \ - - -- ' -

IUF< &+1-- -- ' START-Tt---I-- & ~ - t--- - ~ -- & -- '---- ' -L_ GEN-, ~ I

-

STARTSTARTSTARTSTART

Figure 2.3 Logical diagram of on overcurrent with voltage dependency storting unit

The rneasured quantities of the phase current and voltage are lu , IL2 ,lw and Uu , UL2,UL3,The quantities UU2' UL23 and UWI are used to measure the voltage between thecorresponding two phases.START-R (I generates a starting signal when in phase R the current exceeds the value 1>.All the start ing signal in the phases shall generate a GEN-START signa!. When the earthcurrent exceeds the value IE>, the option IE> is foreseen to generate a GEN-START.With using of the option LO, a low-ohmic earthed system, it generates a start signal in thecorresponding phases if the phase voltage is lower than UF


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The expressions to perform a start signal i n p ha se Rare:PHASE-R = START-R(L)

V (/F>R A/F>S A UFT A/F>R A UF


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The conclusions will be presented at the end of the illustrations. To determine the faultirnpedance the theory of syrnrnetrical components is used where possible. The principles ofthe syrnrnetrical cornponents are described in appendix A. Double-phase fault to earth

Iz.z.

._---Dis!ance relay

-- + Ir : !.Er:

A simple presentation for a double fault to earth is shown in the next figure. In this case it isassurned that the fault is between the phases S en T.Souree Z.

Figure 2.4 Double-phase fault te earthThereby is:Zs :source irnpedanceZe :impedance to earthIR :current in phase RZ :line irnpedanceD'R :voltage on the protection location in phase ROR :voltage on fault location in phase R

The conditions of a double-phase fault to earth at fault location are:

Ils = UT = 0lR = 0 (2.1)Using the syrnrnetrical components:

Ils = dU I +aU2 +1kUT = aUJ +dJL +1k

lR = lJ + l2 + Io(2.2)

Where UIl Uz, UOI 11' hand 10 are the component voltages and currents with normal, inverseand zero quantities denoted by subscripts 1,2 and 0 respectively.



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es Ing of a distance protection system

Combining (2.1) and (2.2) the next equations results in the component values of the voltagesen currents at fault location:

aZUJ + aUz + !l..o = aU/ + aZUz +aZUJ + aUz =aU] + aZUz(aZ - a)UJ = (aZ - a)UzUJ := Uz(2.3)

UJ = Uz = ll..oD+lz+ln=O

According to the two equations above the normal, inverse and the zero network must beconnected parallel. In figure 2.5 this connection is illustrated.

Inverse network

Normal network

Zero network

~ - : - i __I II I

~ 'b I, I Icr:r--ul ~ ~ C : ' __

Figure 2.5 Component-network double-phase fault to earthThe component values of the voltage on the protection location are:

u/ = U/ + l/ZJUz = Uz + LZzIk = ll..o + Ik

The distance protection measures the voltages U'R' U's en U'T- This stands for:!LR = U,+IL+1LJ

1l.s =etu /+aIlz + !l...nUT =aU / + etIlz + !l...n

Principle of distance protection

(2.4)

(2.5)

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Now the possibilities of measuring depending on the sort of fault will be discussed.

measuring between the two phasesTo determine the impedance between two phases the measuring unit uses the differencebetween the voltages and currents of the disturbed phases. In this case the voltages are U'sen U'T' It is assumed that the normal impedance ~ is equal to the inverse impedance ~ 2 Byusing the equations (2.4) and (2.5) the component values can be obtained as follows:

1l.s - O'r = (a 2- a)(U, + Lb.,) + (a2 - a)(U2+ l2b./) + 1l..o-1l..o1l.s - O'r = (a2 - a)(U, - U2) + (a2 - a)(IZ, - IZ ,) (2.6)Il.s - O'r = Zla2 - a)(D -12)

The distance protection measures the currents !R' !s en h. This stands for:IR=L+12+1oIs =a2l/ + al2 + 10Ir =aL + a2l2 + 10

Dividing the voltage by the current:Il..s - !Lr - z- /ls -lr -

Measuring between two phases provides the correct fault impedance.

measuring between phase and earth

(2.7)

(2.8)

(2.9)

The distance protection measures only the impedance in for example phase S, so it measuresU's and!,s. U's can be determined by using the equations (2.1, 2.2 and 2.5).1l.s = a2 0', + alL + ll..n

1l.s = a2(U, + Lb.,) + a(U2+ 12b.) + lb + I k (2.10)1l.s =a2U, + aU2+ alb + a21,Z} + aI2b., + LZoTaking into consideration that the voltage at fault location 1Is = 0, a2+a+l=0 and U1=U2=UOthe equation can be transferred in the following.

(2.11)

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Combined with the sum of the currents:lR + L + lT = IJ = 3 * In

the impedance can be determined by the following equations:1ls = Zla 2D+ al2) + Lk

Jls = Z la2 l 1 + al2 + ~ + In(Zo - ZI), ~ Zo-ZI!l..s = ZIL + L!- ( 3 ), ", Zo - ZIILs = ZiL + L!- ( 3Z ))_I

Zo-ZIwith earth factor k =( 3Z- )1ls =ZiL + kLJ)z - Il.s- I - Is + kLi

To estimate the impedance it is necessary to correct the current by earth factor k.

Phase to phase fault

(2.11)

(2.12)

A simple presentation for a phase to phase fault is shown in the next figure. Also in this faultsituation it is assumed that the fault occurs between the phases S en T, but in this casewithout connection to earth.

- - - . - ,-1. :!.La:

- IT : !lT:Distance relay

!l..r---

1-

Figure 2.6 Phase ta phase faultThe conditions at fault location are:

(2.13)



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The calculations are almost the same as for double-phase faults to earth, but in this situationthe zero-network is not included. Only the important steps of the symmetrical components.wiH be shown.

z.,.s,~ ~ ~ ~ ! - - 1 I

1 1I II 1

1I111

' -------------- - - - - _,

Normal network

Inverse network

Figure 2.7 Component-netwerk Double-phase fault

Now the possibilities ofmeasuring depending on the sort of fault will be discussed.

measuring between the two phases

(2.14)

To determine the impedance between two phases the measuring unit uses the differencebetween the voltages and currents of the disturbed phases. In this case the voltages are U'sen UT' It is assumed that Zl = Zz.

Ils - U ' T= (a 2 - a)(UI+ elI) + (a 2 - a)(U2 + lZI)Ils - U'T= (a 2 - a)(UI - U2) + (a2 - a)(lZI - lZ I )Ils - U 'T= Zla2 - a)(D -l)The distance protection measures the currents IR' 15 en h. This stands for:

lR = II + l2Is = a2L + al2IT = all + a212

(2.15)

(2.16)Dividing the voltage by the current:

I ls- IlT_ 2 IIs -IT - (2.17)

Measuring between two disturbed phases provides the correct fault impedance.



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Testing of a distance protection systemmeasuring between phase and earth

In this case no fault current related to the fault location occurs. Therefore it is not correct tomeasure the voltage and current between phase and earth. Double-phase fault to earth at different places (cross-country)This kind of faults occurs mostly in systems without earthing or with earth faultcompensation. In case of a one-phase fault the fault current may be very smal!, but thevoltages of the non-disturbed phases will be increased till the value of the line voltage. Dueto the higher voltages in the non-disturbed phases an one-phase fault may be occurred in another place.A simple presentation for a double fault to earth at different places is shown in the nextfigure. In this case it is assurned that the fault occurs between the phases S en T.

z.

z. Zv

1- Ir : J.l:T:

'-----,Dislance relay

z. UT

Figure 2.8 Double-phase fault to earth at different places (cross-country)Thereby is:z'v :extra impedance caused by the fault at the other place in phase SZE :earth impedance between two faults

To examine the two possibilities of measuring it's very complex to use syrnrnetrical components. To explain the impedance calculation principally a simpIe electrical network is used.

- 14-Principle of distance protection


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Testing of a distance protection systemmeasuring between the two phases

To illustrate the measuring fault in case of a cross country the next equivalent circuit is used._ Is Z

ZFigure 2.9 Equivalent circuit cross-country

To calculate the impeance the next voltage and l : l l t tnt are essential;ls = -fT = If,lr;- UT = U

The total impedance can be expressed as:U2Z'+Z'v+ Z'E= I

(2.18)

(2.19)

When determining the impedance between the phases the measuring unit calculates thedifferences of the voltage and current between the disturbed phases.Il.s - UT2Z+Z'E+ZV= I

Z = Il.s - llT _ ZE _ Zv (2.20)- 21 2 2lls - Ur ZE Zvz= ----Is - lr 2 2

Il.s - Ur ZE ZvIs-lr =Z'+ 2 + TThe distance protection ca1culates not only the line-impedance Zv but also half of the extraline impedance Zv and earth impedance ZE' The distance protection may detect the fault inthe backup protection zone instead of the protection zone and generates a delayed Trippingtime. That means that the distance protection isolates the faulted circuit with such delay thatthe system could be damaged by the existence of a high short-eircuit current in case of twoone-phase faults to earth.



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.l.!.

Testing of a distance protection systemmeasuring between phase and earth

To take this extra impedance into account the earth current has to be corrected by the earthfactor. This is only possible when measuring between phase and earth which has alreadybeen discussed. Double-phase fault to earth with crossing-resistanceTo examine the influence of a crossing-resistance Rf the important steps of the symmetricalcomponents will be shown. A simple presentation for a double fault to earth with crossingresistance is shown in the next figure. Also in this case it is assumed that the fault oceursbetween the phases S en T.

Souree

Distance relay

Figure 2.10 Double-phase fault to earth with crossing-resistanceThe conditions are:

1ls = UT = (Is + Lr) *RfLR = 0 (2.21)

Using the symmetrical components of equation (2.2) the conditions of the component valueson the fault location are:

U, =U2I, + I 2 + L =0The total earth current is (using equation 2.7):

LR + Is + Lr =3 *L

(2.22)

(2.23)The zero-voltage Uoat the fault place can be obtained by using the equations (2.2), (2.21) and(2.23). The equation a2+a=-1 is used.

1ls=a2U, + alL + Ik =3LRt(a 2 + ajU, + Ik =3LRt (2.24)Ik =3LRt+ U,- 16-Principle of distance protection


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To calculate the zero-voltageUa at the distance protection equation (2.4) is used.Il.o =Ilo + Ik

1l..o = UI + 3f.oRf + LZoU'o = UI + LlZo + 3Rf )

The equivalent circuit for this double-phase fault to earth is given in figure 2.11.

(2.25)

Normal network

Inverse network

Zero network

~ L : L : i __ II IZs, b !, I I

ICJ L J - - - - - : I

~ l l ' ~ !I I- - -1- -II I

3 ~ r : L : ' __iFigure 2.11 Component-network double-phase fault to earth with crossing-resistance

Now the possibilities of measuring depending on the sart of fault will be discussed.

measuring between the two phasesBy measuring between the two phases the zero-component does not participate. So in thiscase the crossing-resistance has no influence. For details about determining the impedancesee the system fault double phase to earth.

measuring between phase and earthComparing with the double-phase fault to earth an extra impedance must be taken intoaccount. When using the same steps as in equations (2.10) and (2.11) the voltage at theprotection location can be determined.

Il.s =a21}ZI + a l ~ 1 + LlZ + 3Rf ) (2.26)



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IZ..s = Zla2L + a12) + li 'k + 3Rf )IZ..s =Zla211 + al2 + k) + k('k - ZI + 3 ~

, 'k - ZIIl....s = Zls +Il ( 3 + Rf), 'k - ZI Rf!ls = Zlls +Il ( 32 + Z ))_I _IIls = Zlls + (k + i ) I I )

_I

To estimate the impedanceIls

2/=-------- RIs + (k + f) 'LI_1

(2.27)

(2.28)

the current has to be corrected by the earth factor k and by a factor which includes thecrossing-resistance. For this reason the R-X plane of the protection zone or back-up zone isnot just a line but contains a certain area with a reasonable reserve for the high-ohmiccrossing resistance Rf. Conclusion measurementsThe next table gives a short result of determining the impedance at double-phase faults.

Table 3.1 Conclusions measuring double-phase faultsSituation Measurement

phase-phase phase-earthDouble-phase fault to earth ok okPhase to phase fault ok not possibleDouble-phase fault to earth at different places (cross-country) not ok okDouble-phase fault to earth with crossing resistance ok ok

With this table the function of the phase selection unit will be explained.2.2.2 Phase selection unitDepending on the starting signaIs of the start ing unit the phase selection unit connects theappropriate voltage(s) and current(s) for determining the impedance between phase andearth or between two phases.The function of the phase-selection unit is to choose phase to phase or phase to earthmeasurement. During a double-phase fault to earth (with or without crossing resistance)both choices are possible, but the unit chooses phase-earth measurement. In case of phase tophase fault there is no earth current so only measuring phase-phase is possible.



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A cross-country fault is a case which mostly occurs in a petersen-coil or a ungroundedsystem. The starting unit should detect undervoltage in both disturbed phases by measuringthe line voltage. When the earth current exceeds a certain value in accordance with thelogical diagram in figure 2.3 the starting unit will as programmed generate a starting signaIfor the disturbed phases and for earth. The phase-selection unit translates this into phaseearth measurement. Now it takes the extra line impedance and earth impedance intoaccount by correcting the earth current.In a petersen-coil or a ungrounded system it is allowed to keep one-phase fault in system.The voltage to neutral is shifted up with the phase voltage but the fault current will be verysmalI. Mostly for overhead lines, where often one-phase faults occurs, this system type isused.To select the faulted circuit there are different facilities to be programmed:

Normal acycle (L3 before Ll before L2) Normal acycle (Ll before L2 before L3 before Ll) Inverse acycle (Ll before L3 before L2) Inverse acycle (Ll before L2 before L3 before Ll)

In appendix B the output of the starting unit for all kind of faults and the correspondingphase selections are shown. In the next paragraph the impedance unit which calculates thefault impedance from protection location to the fault location and its direction will bedescribed.2.2.3 Impedance unitThe impedance unit ca1culates the impedance by using the voltage(s) and current(s) of thephase selection unit.The impedance between two phases can be determined by using:

Z(L _ L) = !l(L - L)- leL - L)Here is Z(L-L) the impedance between the phases, U(L-L) and !(L-L) the voltage and current.If a earth fault appears the calculation is as follows:

U(L - E)l iL - E) = HL -E ) + klE

Here is Z(L-E) the impedance between phase and earth, U(L-E) and !(L-E) the voltage andcurrent in the corresponding phase. As mentioned before it is necessary to make a correctionwith the earth factor 1, which value is complex. To calculate the impedance the values of tevoltage and the currents must be split into real and imaginary part. This will be illustratedfor an fault between the phases S en T.



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Voltage Uz for phase fault between S and T:Re{UJ = R e { ~ } - Re{UT}Im{Uz} = I m { ~ } - Im{UT}

Current!z for phase fault between S and T:Re{lz} = Re{ls} - Re{lT}Im{lz} = Im{ls} - Im{IT}

The algorithm for the signaI processing is calIed Discrete Fourier Transformation (DFT) andwith this algorithm the real- and imaginary part of the signal with fundamental frequencycan be ca1culated. The equation for processing a current signal for instance will be asfollows.The real and imaginary part:

2 n - 1 k2Re{l} =-Lik c o s ( ~ )n k=o n

After adjusting the appropriate values of the voltage(s) and current(s) according to the sortof system fault the resistance and reactance can be calculated as follows:Re{UJ Re{lz} + Im{Uz} Im{lJ

R = (ReUz}j2 + (ImUz}j2x = Im{llz} Re{lz} + Re{Uz} Im{lJ

(ReUz})2 + (Im{lz}j2From the corresponding currents and voltages the direction unit determines the angle whichis a criterion for the direction of the energyflow.After calculating the fault impedance the distance protection compares itwith an establishedtripping characteristic of the protection zone. If the calculated fault impedance is inside theR-X plane of the protection zone the distance protection should put out the line fast. I f theca1culated impedance is outside the R-X plane of the protection zone the fault is located inthe back-up zone and the distance protection should put out the linewith delay.



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2.2.4 Tripping unitTo represent the impedance of the protection or backup-protection zone mostly the nextpolygon tripping characteristic on a R-X plane in figure 2.12 is used.The crossing resistance at the fault location has generally an effect that reduces the effectivereach of the measuring unit. The apparent fault location may be outside the protection zoneand the distance protection underreaches. The polygon characteristic has for this case areasonable reserve for system faults with high-ohmic crossing resistance Rf.

jX

As R

pol gonFigure 2.12 Tripping characteristic of the distance protection

Xs and Rs are the setting value of the first quadrant. The value of Xs is given by the lineimpedance ZL with the line angle CPL' In the second and fourth quadrant the polygonaltripping characteristic can be limited by adjusting the parameter 31 and 3z. Totally there arefive impedance setting zones available ofwhich the first one is the protection zone.To determine in which direction (forward or backward) the system fault occurs the distanceprotection has to measure the direction of the energyflow. By using phase displacementbetween the selected voltage and current quantities the direction unit will be able to detectthe direction of the fauIt. The direction line is fixed through second and fourth quadrant inorder to cope also other special fauIt conditions like current transformer saturation.When the starting unit detects a fauIt it generates a General-start to the timing unit and theimpedance unit begins to calculate the impedance. Depending on the result of theca1culation i t makes the decision in which zone the fault occurs. The time between faultinception and the decision where the fault occurs is called the Tripping time. When in figure2.13 the fauIt occurs between zone 1 (Z1) and zone 2 (22) the starting unit will generate aTripping time in tz.



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rt 15 ----

--4 - - - - - - - - - - -13 - - - - - - - - - - . - - - - - - - - - - l

Iz-

Figure 2.13 Time-Impedance characteristicThe Tripping time has five time-intervals and three of these intervals are depending on theimpedance. The Tripping time t4 and ts are independent of the impedance and ts is alsoindependent of the direction.



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3 Performance of distancespecial-faul-t conditions protection atIn fault situations the distance protection system ha s to determine the impedance betweenthe protection location an d the fault location correctly. The estimated fault impedance isbased on normal sinusoidal values of the voltages an d currents. There are situations inwhich the voltages or the currents are no t sinusoidal. In this case the distance protectionma y have difficulty to deterrnine the correct distance of the fault. This chapter describesthree examples of such fault situations. Cu rrent with DC-component Saturation of current transformers Effect of electrical arc

3.1 Current with OC-componentThis phenomenon occurs mostly during the initial stage of the short-circuit in the system.The current is unsymmetrical in respect of the zero line, that means the current contains andecaying component of De. A genera1form to express the short-circuit current ik(t) in a onephase L-R circuit is:

/'-.

idt) = Un [COS(wt + 'V - CPk) - exp{-!...-}COS('V - CPk)] (3.1),JRI +w2U 'tkThereby is:

the sustained operating current neglectedn the maximum voltage of the sourceRkan d Lk the resistance and inductance of the fault impedance'P the angle at fault inception (-Tt/2 '" Tt/2)Cj>k the phase angle between the voltage an d the steady state short-eircuit current:(IDLk)CPk = arctan R:-'tk th e time constant of th e LR-network

- 23-Performance of distance protection at special fault conditions


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Further study of equation (3.1) gives the next situations: '1'- cp" = :Tt/2. The DC-component is equal to zero and the current goes over to thesteady state. In this case the zero passage of the short-eircuit current is at the samemoment as the fault inception.

'I' = ::1t/2, cp" random. In this si tuation the maximum short-circuit current has thebiggest value. Now the zero passage of the voltage is at the same time as the faultinception.The exponential term in equation (3.1) corresponds with the DC-component. The steadystate short-circuit current flows when the time t is infinite. In the next figure an example of ashort-circuit currentwith DC-eomponent ('t,,=10 ms) is shown.

'0

-6

De-component f\, ,f\ f\ f\ (\ f\ f\ f\,

"- "-, , ' .,.- - , ' -, , - , , - - - -,, ' ,,1 , ,..... or , " ,Angle at \ , ,, , :aun inception, \ ,, , , '. .V V V V V V VC t :2 0 40 oe > BO ., 0 ., 20 ., 40 .., Eta .., sC >

. . [ . .. . ", . .. . _J

Figure 3.1 Short-circuit current with De-componentA short-eircuit current with DC-component can bring on the current transformer insaturation. In the next paragraph this behaviour of the current transformer will be discussedin detail.

3.2 Saturation of current transformersFault currents with high amplitudes can set the current transformer (CT) in saturation.Especially when the current contains a DC-eomponent with a certain time constant. Tosimulate this kind of fault ElectroMagnetic Transient Program (EMTP) is used. With TACS, afacility of EMTP, a simulation model to test the performance of the CT can be constructed.



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Saturation:

The maximum appearance Qf the flux per winding of the CT is: lPm := 2lPl + lPr' withlPl is thenormal sweep of the flux, and lPr the residual flux. At saturation the iron core picks up acertain part lPs. The flux to be left is 2lPl + lPr - lPs; this flux flows in the air and through theiron, which has an incremental relative permeability Jlr nearby to 1 during saturation. Theself-induction Lk is direct proportional to Jlr and will be very small during the saturation. Itis important to know that normally, without saturation, the value of Jlr is about 1000.Before a simulation model of the CT is constructed a simple equivalent electrical circuit willbe made where the primary current and voltage are transferred to the secondary side of theCT.

i ' l, u'l ' i2, u2im

Figure 3.2 Equivalent circuit of a current transformer: primary current and voltage, secondary current and voltage: magnetizing current: loss secondarywinding and burden: coupling-winding, reactance

To demons trate the effect of saturation the next specifications of a CT are used:Current ratio: 1000/1Type: 5PlO, where: 5%: inaccuracy at saturation condition

n:=10 (overcurrent-factor)Sn:=20VA, iN=lA

The loss of the secondarywinding and the nominal burden can be determined as follows:

(3.2)



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With this given the maximal double sweep of the flux can be calculated. Integrating fromone zero-passage to the next zero-passage (half-period of the sinus):

2'aI =nlN(R2 +RNS;l2sin(mt) dtsal = nlN(R2 + RN)m

10 *.J2 * 1(0.2 + 20) =0909 v:sal = 314.16 . sWhere:IN is peak value nominal current ofCTC\lsat is the maximaI flux before saturation occurs

(3.3)

The magnetizing current at point of saturation is equal to the inaccuracy in the range ofovercurrent:

100 A

O.SA

Double swe p of the flux

-100 A

Figure 3.3 Characteristic flux and current due to the iron care

(3.4)

Figure 3.3 shows the behaviour of the iron core with consideration of the saturat ion effect.This characteristic is implemented in the CT-model.



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By TACS the electrical circuit can be translated into a simulation model to test theperformance of the current transformer. In figure 3.4 this model is drawn.i') 12 u' ) J


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Testing of a distance protection system(A)

i 2

rI - -- "-..- I '\ "'-, 1 '\1 \1 \t3 \t J Jim

r90 1 /I /

/, 1 /


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(3.5)


3.3 Electrical arcMostly a short-circuit in overhead lines is accompanied by an electrical arc which ha s a non-linear ohmical behaviour. To understand the process of an electrical arc a short explanationwill follow. After that the arc will be simulated by EMTP.The steady state characteristic of the arc describes the dependenee of the arc-voltage on thecurrent. When the c ur re nt c ha ng es slowly the physical processes can accomrnodateimrnediately with the ne w circumstances an d the voltage of the arc will follow the line likein figure 3.6.

Arc voltago (V)

negative slape

Currenl (A)Figure 3.6 Arc characteristic in steady state

The following points gives a sh ort rep ort of the energy flow during the existence of the arc:

At the existing time of th e arc the feed of electrical power to the arc is given by:Pb=Ub*ib

At the same time energy will be transported to the air by cooling-power Pk. The difference between both energies causes increasing or decreasing of thetemperature in the channel of the arc. The power-balance is:

d(bPb =H + { f t (3.6)Thereby is


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In figure 5.4 it can be seen that the voltage of the electrical arc at fault location has anignition peak after passing the zero-line. Also after a half period of the arc there should be anignition peakagain but due to the maximum sample rate of 10.000 Hz of MICRN this hasbeen neglected. It is important to know that the sampling rate of the distance protect ion islower (1200 Hz) than the sample rate so this peak would be neglected anyway.

- 31 -Performance of distance protection at special fault conditions


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4 Test of distance protection under steadystate conditionIn this chapter the starting unit and the measuring unit are tested under steady stateconditions, so the voltages and the currents have no transients. The tests have been operatedwith the equipment of MICRN. For details about the test equipment see appendix C.The so-called regular test is the first test to be performed. It's a test just for one sett ing of thedistance protection but with several kind of system faults. The results are presented in detail.The two most important units, the starting unit and the measuring unit will undergo thistest.The second test is the 'complete test'. This means a test with using different settings of theunit. The starting unit is tested for four different kind of starter characteristics. Themeasuring unit is exarnined under several net conditions like earth factors. In the completetest only the results will be given.

4.1 Setting regular test distance protectionBefore starting the test in the next paragraph the regular setting of the distance protection isshown. Un =100 V (line voltage)

In=lA UP =0.9 .Un =90V (line voltage)=S1.96V (phase voltage)1> =1 . I n =1 A (phase)IE=O.lIn=O.lAIF = 0.5 . In = 0.5 A Earthing system or net type: Low-ohrnic Phase selection : normal acycle L3-Ll-L2 Zone characteristic

Zone 1 Zone 2 Zone 3 Dir. backup Non dir . backupParameterse t 1R (Ohm) 5 10 20X (Ohm) 5 10 20l (0) -27 -27 -27 -452 (0) 117 117 117 135trip time (ms) 60 500 1000 1500 2000

- 32-Test of distance protection at steady state condition


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Oir. backupjX

polygon

Earth factor kParameter set

k 1.0qllC (0) 0.0

4.2 Test storting unit

Non dir.backuptrip time 5

Zone 1 Zone 2 Zone 3

To test the starting unit the program section 'Starter Characteristics' of MICRN is used(except for testing the system fault phase to phase). The 'starter characteristic shows forwhich values of the voltages and currents the distance protection generates a General-start(see chapter 2 for details). For testing the responding time of the General-start results can befound at the regular test of the measuring unit.The software automatically increases the value of the voltage(s) at the protection locationand carries out a single measurement after each step. Ouring a single measurement thesoftware runs through a specified range of the current and determines the exactmeasurements for starting and release. The ratio of starting and release is calIed the resettingratio.During the regular test and the complete test of the start ing unit the ranges of the currentsand voltages are:Current (phase) from 0 to 10.0 A (phase)Voltage (phase) from 0 to 80.0 V (phase)

Because of missing the possibility to test the fault phase to phase with earth automaticallythe program section 'Direct generator value setting' is used. The values of voltages andcurrents must be increased step by step. In this case the measuring of the resetting ratio isnot possible.To est imate the performance the measurements of MICRN are compared with therequirements of the distance protection which are shown below. The conclusions of theperformance will followat the end of the paragraph.



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(4.1)


Requirements starting unit:Toleranees Voltage and Current 5%,for example the toleranees of UF is 90 . 0.05 =4.5 so absolute tolerance:UF : 85.5 .. 94.5 V (line voltage)For the currents the absolute tolerances are:1> : 0.95 .. 1.05 AIE: 0.095 .. 0.105 AIF : 0.475 .. 0.525 AResetting ratio: 0.95 - 0.98

4.2.2 Regulor test of the storting unitThe unit will be tested for four type of faults:

One phase :U, L2, 13 Two phase : U-L2, L2-L3, 13-L1 Three phase : U-L2-L3For these faults there is na phase displacement between the voltage and the current, so theangle of the fault impedance cp =0. The starter characteristic of the one-phase fault and thethree-phase fault shows the phase voltage and current of the disturbed phase(s). In case of athree-phase fault to compare the results with the requirements the voltage must bemultiplied with factor "3. One-phase faultFor one-phase fault with low-ohmic net-type only the next two conditions are valid (seechapter two):

START - R (/UF

The results of the test are shown in the next figure and tabIe. The characteristic shows thephase voltage and current of the disturbed phase.

UlUn

tUF< - - - - -Ub=50V - - - r,- ---..J

III

- - requirement- maximum deviation

Ibl=O.12AI I ll1n

Ib2=1.07AFigure 4.1 Starter characteristic phase L1



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Thereby is:Th1, Th2Uh

: the real IF>, 1>, measured by OMICRON: th e real UF = 15.0 I> = 6.8Deviation voltage (%) UF = -3.9 UF = -3.9

The distance protection ha s voltage d ep en de nc y a t 50 V (UF). According to th e setting(UF=90V) the mistake of th e distance protection is a factor -../3. With a maximum deviation of15% the current does no t comply with the requirement of 5% an d also the resett ing rat iowith 0.94 is no t sufficient. It should be at least 0.95. Phase to phase faultDuring this kind of fault the values of the voltages an d currents on the protection locationwill be increased manual. At a certain value of the voltage the current will be increased. It'sthe s am e w ay as testing automatically.For saving time the step of the current and voltage is no t equal for t he w h ol e range. It is onlyvalid in that area where voltage dependency is suspected. In this example these area are0.45A .. 0.55 A/85V .. 95V and 0.9 .. 1.1 A/85V .. 95V.For a phase to phase fault with low-ohrnic net-type the next three conditions are valid:

START-R (/V (!F>R A!F>S AUFT A!F>R AUF


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Area V=O.O .. SO.OV V=SO.S .. BO.B3VDeviation current (%) IF> = B.O I> =B.ODeviation voltage (%) UF = < 0.56 UF = 0 .56

With a maximum deviation of 8.0 the current does not comply with the requirements. Three-phase faultFor a three-phase fault with low-ohrnic net-type the next three conditions are valid:

START-R (/V (fF>R !l.fF>s !I. UFT !l.fF>R !I. UF =6.4Devlation voltage (%) UF = 3.9 UF = -3.9

With a deviation of 3.9% only the voltage UF complies with the requirements. The completetest will continue this with different setting of the unit.



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4.2.3 Results complete testTo examine-the accuracy for every phase displacement between the voltageand the currentat the protection location the one-phase fault and three-phase fault are tested with differentangles of the fault impedance, cp =0 .. 360 step 30.The differences between the next settings and the regular setting a re the values of thevoltages and currents, the two variabIe net types and the range of the angle of the faultimpedance.

Setting number one/two

Setting number three/four

UF = 0.9 . Un = 90V (line voltage)= 51.96V (phase voltage)

I> = 1 . In = 1 A (phase)IE = 0.1 In =0.1 AIF = 0.5 . In = 0.5 ASetting one: Low-ohmic systemSetting two: Petersen-coil systemAngle fault impedance 0 .. 360 step 30

UF = 0.5 . Un = 50V (line voltage)=28.87V (phase voltage)I> = 1.5 . In = 1.5 A (phase)IE = 0.5 . In = 0.5 AIF = 0.8 . In =0.8 ASetting three: Low-ohmic systemSetting four: Petersen-coil systemAngle fault impedance 0 .. 360 step 30

For these settings the accuracy of the following parameters are tested:- Resetting ratio- UP, IF, b , IE

In the next tables these remarks are summarized for one and three-phase fa ult. Thedeviation of the current var ies a little for each angle of the fault impedance. Therefore anaverage and a maximum is given for the voltage and current.



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Setting number one:This setting has already been tested in the regular test (paragraph 4.2.1) for one phase angleof the fault impedance. In this case the voltages and currents are in terms of averages andmaxima so there may be some differences.

loble 4.1 One-phose foult Low-ohmicOne phase fault Area 1 Area2Net type: Lew ehmie U= 0 .. 50V U= 50.5 .. 80.83Resetting ratio 0.91 0.94Deviation UF< (%) -3.8 -2.8Deviation IE>, average (%) 11.7Deviation IE>, maximum (%) 15.0Deviation I>, average (%) 6.6Deviation 1>, maximum (%) 7.0

The accuracy of the current does not comply with the requirements. During the test thedistance protection has it's voltage dependency at SO V (Ub). According to the setting(UF=90V) the mistake of the distance protection is about a factor "3.In case of a three-phase fault the distance protection measures between two phases, so UF


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Testing of a distance proteetion systemTable 4.3 One-phase fault. Petersen-coil

One phase fault Area 1 Area2Net type: Compensated U= 0 .. 45V U= 45.5 .. 80.83R ~ l ? ~ t t i n g ratio Q,!H Q . ~ 4Deviation UF< (%) -0.9 -0.4Deviation IE>, average (%) 12.0Deviatlon IE>, maximum (%) 15.0Deviatlon I>, average (%) 6.7Deviation 1>, maximum (%) 7.0

The phase voltage in the non-disturbed phases are 57.75 V so the line voltage will be 89.2 V.In figure 4.4 this is illustrated. According to the requirements the voltage dependency iscorrect.

45 V

disturbed ~ h a s e89.2V

57.74 V 57.74 VFigure 4.4 Vector diagram of the phase voltages at one-phase fault

When testing th e three-phase fault th e starting unit will measure also between tw o phases.The software of MICRN gives the phase voltage of one disturbed phase, 5 0 in this caseUb=SOV. To compare the voltage Ub with the requirement is should be multiplied by --;3.

Table 4.4 Three-phase fault. Petersen-coilThree phase fault Area 1 Area2Net type: Compensated U=0 .. 50V U= 51.0 .. 80.83Resetting ratio, averegae 0.94 0.94Deviation UF, average(%) 7.0Deviation IE>,maximum (%) 7.0Deviation I>, average (%) 6.1Deviation I>, maximum (%) 6.2

There are no differences between this test an d the regular test.Setting number three:For one-phase fault the setting of UF< = SOV. During the test the distance protection ha s it'svoltage d ep en de nc y a t 29 V (Db). According to the setting th e mistake of the distanceprotection is a factor --;3 again.



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Table 4.5 One-phase fault. Low-ohmicOne phase tault Area 1 Area2Net type: Low ohmic U= 0 .. 29V U= 29.5 .. 80.83Resetting ratio 0.94 0.94Deviation UF< (0/0) -0.45 -2.2Deviation IE>, average (0/0) 8.6Devlation IE>, maximum (0/0) 9.2Deviation I>, average (0/0) 5.2Deviation I>, maximum (OIo) 6.7

In case of three-phase fault the start ing unit measures between two phases, so UF< =50V(line voltage). Also here the software of MICRN shows the voltage of one disturbedphase, so Ub=28.5V and should be multiplied with -../3 to compare with the requirements.Table 4.6 Three-phase fault. Low-ohmicThree phase tault Area 1 Area2

Net type: Low ohmic U= 0 .. 28.5V U= 29.5 .. 80.83Resetting ratio 0.94 0.94Deviation UF< (0/0) -1.3 -2.2Deviation IF>, average (%) 6.3Devlation IF>, maximum (0/0) 6.6Deviation I>, average (0/0) 5.8Deviation I>, maximum (OIo) 5.9

Setting numberJour: Table 4.7 One-phase fault, Petersen-coilOne phase tault AreaNet type: Compensated U= 0 .. 80.83VResetting ratio 0.94Deviation UF< (0/0) -0.45Deviatlon IE>, average (0/0)Deviation IE>, maximum (0/0)Deviation I>, average (0/0) 6.1Deviation 1>, maximum (0/0) 6.7

For one-phase fault the voltage dependency is UF< =50V (line voltage) and in a petersencoil or a ungrounded system the starting unit will measure between two phases. When theline voltage is smaller then or equal to UF< and the current is IE> (0.5 A) or bigger thestarting unit will generate a start-signal. However during the test when the voltage of thedisturbed phase is zero the line voltage will be at least 57.77V (see figure 4.5). In thissituation no start-signal at IE> will be generated. At I> the starting unit generates a startsignal for all voltages.

u=ov (dislurbed phase)

57.74 V 57.74 vFigure 4.5 Vector diagram of the phase voltages at one-phase fault.



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During a three-phase fault the start ing unit measures between two phases. The softwaregives the phase voltage of one disturbed phase, so in this case Ub=29V. To compare with therequirement ( U F < ~ 5 0 V ) the voltage Uh should be multiplied by "3.

loble 4.8 lhree-phose foult, Petersen-coilOne phase fault Area 1 Area2Net type: Compensated U= 0 .. 29V U= 29.5 .. 80.83Resetting ratio, average 0.94 0.94Deviation UF, average(%) 6.4Deviation IF>, maximum (%) 6.6Deviation I>, average (%) 6.0Deviation I>, maximum (%) 6.1

4.3 Test measuring unitFor testing this unit the program section 'Time grading schedule' of OMICRON is used. Theoutput of this section is the signal Tripping time as function of impedance for each zone andthe signal General-start. To test the distance protection the next test model is used:

Souree1- - - - I. :,,,

Distance protection

Here is:IsZsl:Ks :Zu:Zm:

k:

Figure 4.6 One-phose test model

short-cut current and thus the test current (always 2A)souree impedance; the source impedance is variabie, i t will provide aconstant test currenta factor to provide low-ohmic or petersen-coil systemnormal impedance of the fault impedancemutual impedance:

ZJ -Z IZn=-3earth factor; k= 1.0 + j 0.0 Ohm or Ik 1=1.0; CPk=O.ZJ - ZI Zn

k = 3Z/ = ZI

Test of distance protection at steady state condition - 41 -


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This model is drawn for a one-phase system. The test current or short-circuit current is fixedby a constant value of 2A. Therefore the source impedance will fit its value for every changeof the fault impedance.The software automatically increases the value of the impedance Zu from -1.0 to 22.0 Cl witha step of 0.02 Cl and carries out the responding time of the distance protection after each stepand enters the impedance and time in a graph. To estimate the performance the graph iscompared with the requirements of the distance protection which are shown below. Theconc1usions of the performance will follow at the end of the paragraph.The requirements of the measuring unit are:Impedance : 2%Tripping time: 3%Example For zone 1

Maximum difference impedance is 0.02*5==0.1 Ohm soabsolute toleranee impedance is 4.9 .. 5.1 Ohm.

For the Tripping time the absolute tolerance is 58.2 .. 61.8 seconds.During the regular test the time-impedance characteristic is tested for just one setting. A one,two and three-phase fault are illustrated just one angle of the fault impedance , cp == 0.

4.3.1 Regular test measuring unitDuring each impedance step OMICRON measures the Tripping time. I f the measuring unitgenerates no Tripping time this will be remarked in the time-impedance characteristic. Theresults will also be shown in a tabIe, but here only the deviations of the borders are given.The directional backup zone can not be tested automatically, so this is tested manual atrandom fault impedances.The measuring unit undergoes the one-phase, phase to phase and the three-phase fault. Formeasuring automatically this is also the limit of the MICRON equipment. A review aboutthe differences between the requirements and the results is given at end of the paragraph. One-phase faultWhen detecting a one-phase fault the unit measures, in case of low-ohmic net type, betweenthe disturbed phase and earth (see chapter two). In the next figure and table the results aregiven.



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r1IS

1.500 t4

1.010 13

508.78 mo 12

71.s8mo t,

I- - - - -II

- - II ~ - - - - II -I, - - -r- II

I I I- - - -- - - - - requirement- - maximum deviation

Z1 2l!4.938 n 9.837 n 19.688 n z--+Figure 4.7 Time-Impedance characteristic one-phase fault in L1

Zone Impedance border (Ohm) Deviation (%) Tripping time (ms) Deviation General start (ms)-- .. -Z1 4.938 -1.25 71.58 11.58 ms 57.19

Z2 9.837 -1.63 508.78 1.76% 59.59Z3 19.662 -1.56 1010 1.08 % 62.68

Dir Zone - - 1500 0.0% - Phase to phase faultWhen detecting a double-phase fault the unit measures between two disturbed phases.Results are shown in figure 4.8 and in the tabIe.

rIts

72.98 mo t, f - - - - - - - - - l

1.500 14

1.00 0 t3

513.77 mo 12---:---r-

- - - - - - requirement- , . . , . , - . , ---- ,J I-r I --- maximum deviation

I IZ, 2l!

4.988 n 9.938n 19.838 D Z --+Figure 4.8 Time-Impedance characteristic double-phase fault in L1and L2Zone Impedance border (Ohm) Deviation (%) Tripping time (ms) Deviation General start (msZ1 4.988 -0.25 72.98 12.98 ms 60.99Z2 9.938 -0.63 513.77 2.75% 54.79Z3 19.838 -0.81 1000 0.40% 56.79

Dir Zone - - 1500 0.0% -



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Three-phase faultWhen detecting a three-phase fault the unit measures between two disturbed phases. In thenext figure and table the results are given.r1

15

1.50. 14

1.01. 13

510.78 m. 12

68.38 ms 11

f-- - - - ,II- -

If - - I -f - - ; - - - -r III I I- - - -

- - - - - requirement- - maximum devlq

4.9620 9.9130 19.8380 Z---+Figure 4.9 Time-Impedance characteristic three-phase fault in L1. L2 and L3Zone Impedance border (Ohm) Deviation (%) Tripping time (ms) Deviation General start (ms)Z1 4.962 -0.75 66.08 6.08 ms 58.79Z2 9.913 -0.87 510.78 2.16% 61.58Z3 19.838 -0.81 1010 0.61 % 60.79

Dir Zone . . 1500 0.0% -Ouring the tests of one, two and three-phase faults the deviation of the impedance andTripping time complies with the requirements, except in zone 1. In that case the deviation ofthe Tripping time is 12.98 f iS at the most. Comparing with other zones it's not very high safor the complete test the deviation of the Tripping time in zone 1 will not be marked.Ouring the complete test the results of the next faults are shown.

One phase : U, L2, L3 Phase to phase : U-L2, L2-L3, L3-L1 Three phase : U-L2-L34.3.2 Results complete testNow the measuring unit will undergo five tests. For the first three tests one setting of thetime-impedance characteristic will be changed. The tests are:

- Changing the earth factor- Changing the impedance limit of zone 1- Test tripping characteristic by investigating the direction unit- Investigating the voltage memoryThe test will be executed for one-phase fault, phase to phase fault and three-phase fault. Theunchanged settings are equal to the regular setting.



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Angle fault impedanceNow all type of faults will be tested with several angles of the fault impedance


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Limit zone 1Now zone 21 is variabIe from 0.1 to 5 Ohm. When testing the unit with a setting 21=0.1 Qthe conclusions are:One-phase fa uIt Ll

Zone Impedance border (Ohm) Deviation Tripping time (ms) DeviationZ1 1.191 1.0910 78.08 18.08 msZ2 9.837 -1.63% 513.97 2.79%Z3 19.662 -1.69 % 1020 1.51%

Dir Zone - - 1500 0.0%When the fault impedance is smaller than 1.191 Q the Tripping time is 2.0 s. This is theresponding time in the non-directional zone 5. When the fault impedanee is bigger than1.191 Q the unit responds correct. The deviation of 1.091 is caused by the setting 21=0.1 Q.Double-phase fa uIt Ll-L2

Zone Impedance border (Ohm) Deviation Tripping time (ms) DeviationZ1 0.112 0.0120 7 70.48Z2 9.938 -0.63 % 512.18 2.44 %Z3 19.838 -0.81% 1010 0.74%

Dir Zone - - 1500 0.0%With the setting 2 1=0.1 the deviation of the impedance and the Tripping time are acceptable.Three-phase fa uIt Ll-L2-L3

Zone Impedance border (Ohm) Devia tion Tripping time (ms) DeviationZ1 2.942 2.8420 72.98 12.98 msZ2 9.913 -0.87 % 511.18 2.24%Z3 19.838 -0.81 % 1010 1.04%

Dir Zone - - 1500 0.0%Compared with the one-phase fault the only difference is the deviation in zone 1. In this casethe minimum l imit for zone 1 is 2.942 Q.



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Direction unitFor this test the borders of the zones in the second(test A) and fourth quadrant (test B) areexamined. The next picture shows this.

R

polygonFigure 4.10 Testing border tripping characteristic

Test A is executed from 2= 116 to 121, step 0.2. Test Bis executed from 1= -22 to -27,step 0.2. The next table reports the border and the deviation of the two quadrants.Table 4.11 Results testing border

Angle border (") 119.6 -23.4Deviation (%) +2.22 -13.33Deviation, absolute (") +2.6 -3.6

Voltage memoryTo test the sensitivity of measur ing the direction (voltage memory) the voltage at theprotection location (in forward and in backward direction) must be very small (=OV). Thefault inception must appear after 100 ms of nominaI voltage and current. When the test is'OK' the measuring unit reacts in the correct direction and complies with the requirements.The table shows the results.

Table 4.12 Test voltage memoryVoltage memoryDirection Forward BackwardOne-phase fault OK OKPhase to phase faul OK OKThree-phase fault OK OK

The voltage at the protection location is 0.01 V. The modulus of the fault impedance is 0.002n. The direction of the short-circuit is in forward direction 45 and in backward direction 135.



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4.4 Test results under steady state conditionStarting unit

Only in case of detecting the overvoltage it complieswith the requirements. Todetect the overcurrent themaximum deviation in percentage terms is 15%, theabsolute value is 0.015 A. The definition of UF< is confusing. When measuring between two phases the testcorrespondswith the setting, but in case of measuring between phase and earth thevalue UF< is a factor --/3 smaller.

Measuring unit

According to the regular test of the measuring unit the signal General-start starts atcirca 60 ms after fault inception and with that it complies with the requirement. Inzone 1 the tripping time (= 70 ms) does not complywith the requirement, butcomparing with the other zones the value of the absolute deviation is almost thesame.

During the test of the time-impedance characteristic the measuring unit does notcomply with the requirements in the next cases:-


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Testing of a distance proteetion system

5 Test of distance protection under transientconditionThe fundamentals of distance-to-fault measurement are completely involved with thecurrent and voltage signals available at the protection location. Since the measuring unitreceives its signals from the secundaries of current and voltage transformers, these latteritems have an important influence on the dynamic performance of a distance protection.In this chapter the algorithm of the distance protection to calculate the impedance will beexamined. In paragraph 2.2.3 it is shown that the fundamental frequency components ofboth voltages and currents are obtained by using the algorithm Discrete Fourier Transform(DIT). The ratios of appropriate voltages and currents then provide the impedance to thefault. The performance of this a l ~ o r i t h m is dependent on obtaining accurate estimates of thefundamental frequency components of a signal from a few samples.50 in some short-circuit situations with non-fundamental frequencies (transients) it canhappen that the distance protection is not in position to select the faulted circuit or isolatethe faulted circuit correctly. Therefore several {ault situations are simulated by thecalculation program EMTP. The output of this program, which contains data of the voltagesand currents at the protection location, is translated in a data format data which can be usedin MIRON. To translate the output of EMTP to MICRN the software program' translator' has been developed. The connection between EMTP and OMICRN is drawn infigure 5.1.

Data voltage and currentat protection location

forOMICRON

jyd _> /

'" ~ ; g " a . voltage aM\ : ~ ~ ~ n t for simulating;$i the fault situation:V

- ~ ; . . . yFigure 5.1 Connection between EMTP end OMICRON

ault s1tuation

Data voltage and currentat protection locatiofrom EMTP

After the translation the voltages and currents of fault situations calculated by EMTP can besent to the distance protection.

Test of distance protection under transient condition - 49-


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To test the distance protection the next fault situationswith transients will be tested: Fault currentwith DC-component Saturation of current transformer Electrical arc at fault location Change of current direction Cross-country fault

For each fault s ituations two signaIs of the distance protection, the General-start and thetripping time, will be recorded by MICRN. To test the distance protection the nextlimitations are applied: According to the test of distance protection under steady state condition (chapter 4)the behaviour of the distance protection is for each type of system fault almost thesame. Therefore the fault situations (except the cross-country fault) are tested forjust a one-phase fau1t, a two-phase fault and a three phase fault. To test the distance protection only a fault in zone 1 will be considered. For each fault situat ion the test results shows the signals of the General start and theTripping time.According to the resu1ts of testing the measuring unit (paragraph 4.3.3) in zone 1 theGeneral-start signal has been generated at a maximum of 63 rns after fault inception and theTripping time signal at 75 ms after fault inception. For the five fault situations therequirements for locating the fau1t in zone 1 will be less critical: General-start: smaller or equal to 65 rns

Tripping time: smaller or equal to 90 rnsThe settings of the distance protection are for all tests the same and have already been usedin the complete tests of the starting unit and the measuring unit in chapter 4. These settingwill be repeated below. Un = 100 V (line voltage)

In= 1 A UP = 0.9 .Un = 90V (line voltage)= 51.96V (phase voltage)I> = 1 . In = 1 A (phase)IE=O.lIn=O.lAIF = 0.5 . In =0.5 A Earthing system or net type : Low-ohrnicIn case of a cross-country fault the system is earthed by a Petersen coil Phase selection : norrnal acycle L3-L1-L2 Zone characteristic

Zone 1 Zone 2 Zone 3 Dir. backup Non dir. backupParameterset 1R (Ohm) 5 10 20X (Ohm) 5 10 200\ (0) -27 27 -27 -4502 (0) 117 117 117 135trip time (ms) 60 500 1000 1500 2000



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.Non dir.bac.!!ul1

polygon

R

trip time 5Dir. backup

Zone 1 Zone 2 Zone 3

Test circuit:

,,, '__ _ I

Distance protection

Here is:IsZ51ksZuZm

k:

short-eut current and thus the test rurrent (always 2A)source impedance; Z51= 0.58 + j 19.8 Ohm or IZs11 =19.8; CPs=88.a factor to provide the earthing systemline impedance; Zu= 0.88 + j 5 Ohm or IZul =5.08; CPL=80.mutual impedance:

ZJ - Z/z , ,= - - 3earth factor; k= 1.0 + j 0.0 Ohm or Ik I=1.0; CPk=O.

ZJ-Z/ z"k = 3Z/ = Z/

In the following paragraphs the tests of the five fault situations wil! be tested for three typeof system faults. Each paragraph contains the method of testing, a detailed discussion of aone-phase fault and the results of all type of system faults. Details about the first three faultsituations are given in chapter 3.

Test of distance protection under transient condition - 51 -


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5.1 Fault current with De-componentThe OC-component of a short-circuit current is completely unrelated to the distance fromprotection location to the fault location, however i t depends upon the instant in the voltagecycle at which the short-circuit occurs. If the short circuit occurs when the voltage is nearbythe zero-passage the OC-component is a maximum and the fault current signal is initially afuUy offset eosine wave. If the shor t circuit occurs at the top of the voltage the OCcomponent is zero and the fault current signal is a pure sine wave. The DC transientcomponent can be an important souree of error, which tends to change the impedance seenby the distance protection.5.1 .1 MethodThe decay and the amplitude of the OC-eomponent is dependent on the L/R time constantof the line model. A higher value of the time constant causes a faster decay and a higheramplitude of the OC-component. Therefore the distance protection will be tested for a timeconstant with 'tk=10 ms and 'tk=50 ms.5.1 .1 Discussion testing of one-phase faultIn this paragraph the test will be illustrated for one system fault, namely the one-phase faultand the L/R time constant of the line 'tk=lO ms. The signals of the voltages and currents atthe protection location are shown in figure 5.2. The signals General-start and Tripping timeare shown at the bottom.

-u- - I

Tripping timeGeneral start f - -------------------c=====f

L1

L2

L3

o 60 t (ms) 120,

lBO

Figure 5.2 Voltages and currents ot protection location tor one phase tault



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The effective value of the susta ined single-phase short-circuit current is in case of a lowohmic earthed system with ks=k=l:

I = UN (phase).< (1 + ks)Zs, + (1 + k)Zu 100/.JJ= 2(19.8 +5.08) =1.16A (5.1)

The one-phase fault in phase L1 occurs at 100 ms. At 61.6 milliseconds after fault inceptionthe start ing unit detects the fault and starts the distance protection. When the distanceprotection has located the fault it generates a tripping signal at 76.1 milliseconds after faultinception.In the next table the maximum responding time of the signals General-start and the Trippingtime are given for four type of faults. The test is executed for a sustained short-circuit currentand a short-circuit current with DC-component. Practical va lues of the L/R time constant ofthe line are between 10 and 50 ms, and therefore the tests are executed for a time constant of10 ros and 50 ms.

Tabla 5.1 Raspond distance protection tor tault current with De-component.DC-component General start (ms) Tripping t ime (ms)

System faultOne phase no 61.6 84.S

10 ms 56.6 76.1SOms 61.5 71.1

Phase to phase no 58.9 78.110 ms 58.9 68.350ms 57.6 77.0

Double phase to earth no 59.3 88.410 ms 55.5 74.9SOms 60.8 70.3

Three phase no 61.6 84.610 ms 59.5 68.950ms 59.4 69.0

Comparing with the one-phase fault without OC-component the differences in theresponding time are negligible.

5.2 Saturation of current transformerAn other area of concern is the saturation of a current transformer (CT) dur ing an systemfault. A satura ted current, which can be caused by a fault current with OC-eomponent,produces no secondary current while the CT core is in saturation. Typical wavefor rns ofprimary current and secondary current are given in paragraph 3.2 and it should be dear thatthis has it's effect for detecting the fault location.



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5.2.1 MethodTo test the distance protection with the effect of saturation the next specifications of the CTare used to set up the behaviour of saturation.

Nominal power: 20 VANominal current: 1 ACurrent ratio: 1000/1Inaccuracy of the nominal current : 5%Overcurrent-factor : 1Burden : 20.2 Q

This specific behaviour of saturation will be used to for all type of faults. To test the distanceprotection the next steps will be proceeded:1 By a established behaviour of CT saturation the test will be performed for a fault

impedance at the border of zone 1 ( IZu I=5.08 Ohm)2 If the signals General-start and the Tripping time do not confirmwith therequirements it locates the fault outside zone 1. Therefore the testmust be executedwith a smaller value of the voltage at protection location which pretends to expandzone 1. Otherwise if the distance protection locates the fault correctly zone 1 will bereduced to find the limit of zone 1.3 With the new value of the voltage the impedance limit of zone 1 can be determined.

The value of the voltage at protection location can be changed in the software of OMICRON.Namely it is possible to add a multiplying factor in the data file of OMICRON derived fromthe program 'Translator'. Expanding zone 1 can be accomplished by reducing the voltage.That results in a smaller fault impedance and it seems that the area of zone 1 is increased insize. The change of the voltage is inversely proportional with the impedance of zone 1.5.2.2 Discussion testing of one-phase faultIn this paragraph a one-phase fault in phase L1 with saturation of the current transformerwill be discussed. The cause of saturation is mostly the DC-component in the fault current atthe primary side of the CT as discussed in paragraph 5.1. The high peak of the currentgenerates a high value of the flux in the core. This results in a high magnetizing currentwhich causes subsequently differences between the prirnary current and secondary currentof the CT. The secondary current and the voltage at the fault place are given in figure 5.2.



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Testing of a distance protection system- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

-u- - ITripping timeGeneral start f--------------------c====j

L1

L2

L3

o 60 120 180Figure 5.3 Voltages ond currents ot protection location for one phase fault

To recognize the fault at the border of zone 1 the value of the voltage must be multiplied by0.6 which reduces the fault irnpedance with the same proportion. This results in generatingthe signal General-start at 61.6 ms and the Tripping time at 71.0 ms after fault inception. Sobasically to locate the fault in zone 1 the distance protection underreaches from animpedance of 3.05 n (0.6 . 5.08). In the next table for each type of system fault themultiplying factor for the voltage and both signals General-start and Tripping time areshown.

Table 5.2 Respond distance protection during saturation of CT.DC-component U-factor IZI underreach (Ohm) General start (ms) Tripping time (ms)

5ystem faultOne phase na 1.0 5.08 58.3 77.6

10 0.6 3.05 61.6 71.0Phase to phase na 0_8 4.06 69.4 78.8

10 0.9 4.57 64.4 64.6Double phase to earth no 1.0 5.08 58.6 78.1

10 0.6 3.05 56.9 76.4Three phase no 0.5 2.54 61.8 81.410 0.5 2.54 61.3 80.4

The test is executed for a sustained short-eircuit current and a short-circuit current with DCcomponent. The test in paragraph 5.1 shows the same behaviour of the distance protectionfor both t ime constants, 10 and 50 ms. Therefore only the t ime constant with 't=10 rns isexecuted to test the saturation of the CT. As expected the distance protection underreachesas the current transformer is in saturation.



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20

" 1 5


5.3 Electrical arc at fault locationThe third transient is the electrical arc at the fault location. It causes a higher voltage at thelocation of the distance protection and generates also an extra re-ignition peak at the timethat the current is nearby the zero-line. The extra voltage at the protection location has a biginfluence on determining the fault location. It tends to expand the impedance seen by thedistance protection.5.3.1 MethodTo test the influence of the electrical arc on the performance of the distance protection an arcmodel as shown in figure 5.4 will be added to the fault location. In this example the arcvoltage has a re-ignition peak at 16 V after passing the zero-line of the current. At peak valueof the current the arc voltage will be reduced to 12.3 V.

re-ignition peak at 16V Amplitude arc voltage: 12.3V/ /

" 1 0

_-----.Y-------_o_5

-"10

-"15

-20 , \ v A \ ~ - - - - - - - - - - - - - - -"165 "170 "175 "1S0--- Electric arc at fault location

- - - - - - - - Arc currentFigure 5.4 Test model eleetrieal are with re-ignition peak at 16 V

By increasing the arc voltage, starting at 0 V, the limit of locating the fault properly will bedetermined. During the test the value between the re-ignition peak and the amplitude willremain its proportion.5.3.2 Discussion testing of one-phase faultIn this paragraph a one-phase fault with electrical arc occurs in phase Lt. In figure 5.5 thevoltages and currents at the protection location are given. The higher voltage in phase L1 iscaused by the electrical arc with a re-ignition peak of 16 V.



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-u- - ITripping timeGeneral start

L1

L2

L3

60 120 lBOt (ms)Figure 5.5 Voltages and currents at protection location tor one phase taultThe height of the arc voltage in this figure presents the limit to locate the fault properly. Thedistance protection generates a General-start at 56.5 ms and the Tripping time at 76.1 msafter fault inception. In the next table the limits are determined for several system faults witha time constants of the line impedance of 10 ms. The limit of the electric arc is given by thepeak value of the voltage at re-ignition of the arc.

Table 5.3 Raspond distance protection during electric arcOC-component U at re- igni tion (V) General start (ms) Tripping t ime (ms)

System faultOne phase no 14.0 57.8 87.2

10ms 16.0 56.5 76.1Phase ta phase no 22.0 64.3 83.7

10ms 23.0 55.1 74.6Double phase to earth no 1.0 64.3 83.7

10ms 9.0 55.1 76.4Three phase no 26.0 56.4 85.9- - 10ms 28.0 55.1 76.4

There are two remarkable occurrences: A fault current with OC-component results in locating the fault location properly athigher values of the electrical arc. In case of a double-phase fault to earth to locate the fault correct the limit of thevoltage at re-ignition is very low, namely 1 volt.



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5.4 Change of current directionIn this paragraph a transient of the short-circuit current occurs by changing its direction 180degrees. When the fault is located in the back-up zone due to the transient the distanceprotection may recognize the fault in the protection zone resulting in a tripping signal to thecircuit-breaker. However the correct behaviour of the distance protection is to generate onlya General-start for detecting the occurrence of the fault outside the protection zone (back-upzone).5.4.1 MethodTo test the influence of 1800 phase displacement the next situation is simulated in EMTP.

Currenl measured byZ dislance ~ ~ o l e c l i o ~ _ __- :

9110 Z -----. l / I OZ+ - - -

I 9110 Z l/lOZCircuit A91l0Z 1I10Z l 200 ms circuil-Source breaker opens Z, Source

Faull inceplion al 100ms

Circuit B Zt---------------1c::=Jt--------+-----t--iZ

, ,'_- 1Proleclion localion

- -- -- - Currenl after circuil-breaker opens-----. Currenl during onephase fault

Figure 5.6 Net situation for change the direction of the currentIn this situation a one-phase fault occurs in circuit A at 100 ms. The current during the onephase fault flows in the same direction as the arrow of the current IpL and indicates thebackward direction of the distance protection placed in circuit B. When the circuit-breakeropens (at t=200 ms) the direction of the current in circuit Bwill be changed.5.4.2 Discussion testing of one-phase faultIn this paragraph a one-phase fault as situated in figure 5.7 will be discussed in detail. In thissituation the fault location is not in the protection zone of the distance protection in circuit B.50 basically when the fault is in the back-up zone the test analyses the sensitivity of thedistance protection by changing the direction of the short-circuit current.Test of distance protection under transient condition - 58-


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In short the test proceeds as follows: From t=O f iS to 100 ms there is no occurrence of a fault. At t==100 ms the first fault inception in phase L1(circuit A) occurs. The distanceprotection should only generate a General-start, because the fault is located in thebackward direction (back-up zone). At t=200 ms the circuit-break opens and the direction of the currents changes 1800and the distance protection should recognize the fault in the forward direction (alsoback-up zone).

To test the behaviour of the distance protection in forward and backward direction moreexplicit the zone characteristic is set as follows:Zone 1 Zone 2 Zone 3 Dir. backup Non dir. backup

Parameterset 1R (Ohm) 10 15 20X (Ohm) 10 15 2051 (0) -27 -27 -27 4552 (0) 117 117 117 135trip time (ms) 100 500 1000 400 2000

The zone 1, zone 2 and zone 3 are set in forward direction and the dir. backup zone is set inbackward direction.In the next figure the situation with direction change of the current is illustrated by thevoltages and the currents at the location of the distance protection.

-u- - ITripping timeGeneral start r-----------c=========================j

L1

L2

L3

o 100 200 300t (ms)

Figure 5.7 Voltages and currents at protection location tor one phase tault



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After the direction change of the current the distance protection should generate a trippingsignal after 500 ms. In figure 5.7 the fault occurs ti11300 ms, so the distance protection shouldonly generate a General-start. Otherwise it may recognize the fault location in zone 1 andgenerates a tripping signal after 100 ms.It is clear that the distance protection responds correctly, because it generates only aGeneral-start at 63.9 ms after fault inception in circuit A and it locates no fault in theprotection zone. Basically the distance protection detects only a fault outside the protectionzone and does not respond during the direction change of the fault current caused byisolating the faulted circuit in the backup zone. In the next table the results for other systemfaults are reported.

lable 5.4 Respond distance protection with direction change of the current.Time constant (ms) General start (ms) Tripping time (ms)System fault

One phase 10 39.5 nonePhase to phase 10 43.2 noneDouble phase to earth 10 34.5 noneThree phase 10 67.8 none

For all system faults the distance protection responds correctly.

5.5 Cross-country faultIn a Petersen coil system in case of a one-phase fault the voltages in the non-disturbedphases can be raised by a factor "3. In spite of the system design for these high voltages theone-phase fault can cause a second one-phase fault in an other line section. When twophases of an overhead line flash-over simultaneously at the same location or within the samesection of line, it is known as a double-phase to earth fault. When two phases flash-oversimultaneously in different line sections, it is known as a simultaneous earth or crosscountry fault. This situation is drawn in figure 5.8.

Test of distanee proteet/on under transient condition - 60-


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Testing of a distance protection system112 Z 112 Z

- Circuit A 112Z112 Z

Source

Circuit B

:Protection locationA Faull inceplion in phase LI al 100ms

,+-- - -, II2Z 112 Z. .

112 Z 112 Z..---JII2Z ii2 Z. .'._--,

Protection location BFault inception in phase L2 al 200 ros

--. Faull currcnl duling a cross-country faullFigure 5.8 Cross-country fault

Burden

In this figure the distance protection is installed in circuit A an d Ban d for each protectionsystem the behaviour during the cross-country fault is examined. The setting of the phaseselection u ni t o f t he protection system is L3-LI-L2 an d provides the sequence of determiningth e distance of the fault in the corresponding phase. In ou r case a one-phase fault occurs inL1 an d L2 so th e distance protection will determine the fault location i n p ha se L l.The test proceeds as follows: From t=O ms to 100 ms there is no occurrence of a system fault. At t=100 ms th e first fault inception in phase L l (circuit A) occurs. In our case th e

system ha s a petersen coi! earthing system 5 0 the distance protection in A and Bs h ou l d n ot respond, because the system remains it's symmetry. At t=200 ms th e second fault in phase L2 in circuit Bbegins. The system is notsymmetrical anymore and the distance protection should recognize the doublephase fault to earth. The phase selection unit of the distance protection selects phaseL l to determine the fault location. At t=300 ms th e system keeps a one-phase fault a nd t he system is symmetricalagain.

In t he n ex t figure the voltages and the currents at protection location A are illustrated.

Test of distance protection under transient condition - 61 -


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Testing of a distance protection systemJ\I \ I ' /\ /I '\1 I /" / \ /i 1 I

UI

Tripping timeGeneral startl-----------------i=====:::J---j

L1

L2L3

o 150 300t (ms)Figure 5.9 Voltages ond currents at protection location Aduring a cross-country fault with petersen-coil

In this situation both distance protections responds correct. During a double-phase fault toearth the distanee protection in circuit A locates the fauIt in phase L1 in its own protectionzone and generates a tripping signal to isoIate the faulted phase as quick as possible. Theprotection system in circuit B recognizes the fault in the back-up protection zone and willgenerate a tripping signal with delay if the distance protection in circuit A fails to trip. Intable 5.5 a small review of the behaviour of the two distance protections are given.

Table 5.5 Respond distance protection during a cross-country faultStart signal lsolating phase by

selecting sequence L3-L1-L2Event circuit A circuit B

no system tault . . .one-phase taul! in A - none noneone-phase tault in B Start-L1/Start-L2/Start-E l1 noneone-phase fault in A Start-L1 none none

If the starting unit generates three start signaIs, namely start-LI, start-L2 and Start-E, thedistance protection recognizes two one-phase faults to earth in phase LI and L2 and thephase selection will select phase LI to determine the distance of the fault. Only the distanceprotection in circuit A recognizes the fault in the protection zone and isolates phase L1 asquick as possible.



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5.6 Test results under transient condition The behaviour of the distance protection is the same for a fault currentwith or

without DC-component. Saturation of the CT tends to locate the fault further on. In case of a three-phasefault the deviation of measuring the fault location is 100%. Also the electrical arc at the fault place tends to locate the fault further on.Comparing with a sinusoidal short-eircuit current a currentwith DC-componentresults in determining the fault location properly at higher values of the electricalarc. In case of a double-phase fault to earth to locate the fault properly themaximum peak value of the electrical arc must be very low, namely 1 V. For a fault outside the protection zone the distance protection did not respond to thechange of the direction of the fauit current, 80 it perfmed correctly. In case of a cross-country fault the distance protection determines the fault locationproperly in accordancewith the setting of the phase selection.



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6 Conclusions and recommendationsTo test the hardware of the distance protection the statie behaviour is examined by the testappliance MICRN. For testing the dynamic behaviour also the simulation programElectro Magnetic Transient Program (EMTP) is used. With this program transient voltagesand eurrents for different fault situations like the electrical arc can be calculated.During the developrnent of the distance protection the statie and the dynamic behaviour ofthe protection system is tested by employing the steady state test and the transient test. Itcan be conc1uded that: in the starting unit there is equivocality in the setting voltage dependency for several earth factors and settings of the zones the deviation of determining theimpedance does not complywith the requirements of 2%. transients caused by saturation of the current transformer and the electrical arctends the distanee protection to determine the fault location f

distance prt

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